This is a proposal to investigate the role of microRNAs in autoimmune diseases. Collectively, 5-10% of the western population is affected by some of the more than 80 autoimmune diseases that arise when the immune system attacks its own body. Current treatment options have limited efficacy, are expensive and have severe side effects such as increased risk for infections and cancer. In the long-term we aim to exploit microRNAs to control the immune system gone awry in patients with autoimmune diseases in order to restore immune homeostasis. Regulatory T cells (Treg) are cells of the immune system specialized to suppress immune responses. Due to their proven ability to prevent and even cure preclinical animal models of autoimmune diseases such as Type 1 diabetes and multiple sclerosis and their important role in preventing rejection of transplanted organs, Treg have entered clinical trials. In contrast, T follicular helper (TFH) cells provide classic help to B clls supporting the B cells' function to produce high-affinity antibodies. Both, dysregulation of Treg o TFH promote autoimmunity. microRNAs are regulators of gene expression only discovered relatively recently. Therefore our understanding of microRNA function in the immune system is very limited. Using in vivo gene ablation we have demonstrated that Treg and TFH depend on miRNA function. Specifically, we showed the importance of the miRNA cluster miR-17-92 for Treg and TFH function. It is intriguing that miR-17-92 regulates both, promoters and inhibitors of immune responses, respectively. We propose to investigate the molecular and cellular roles for miR-17-92 in Treg and TFH. These studies will be performed in cell culture assays in vitro and to some degree in mice. We are using mice that lack the miR-17- 92 cluster specifically in all T cells (including TFH) or only in Treg. Using genome-wide approaches (microarrays, RNA-sequencing) we will also characterize genetic networks regulated by miR-17-92. miRNAs generally regulate hundreds of genes simultaneously. We therefore expect to find multiple candidate genes that will need validation. We will focus on a few genes to study their function in detail in Treg and TFH. Finally, we will test the overall impact of the miR-17-92 cluster in a model of autoimmune disease. The specific aims of this proposal are: 1) Exploring the functional role of miR-17-92 in TFH and Treg in response to the microenvironment 2) Determine the molecular control of miR-17-92 function in TFH and Treg 3) The role of miR-17-92 in autoimmune disease.